1. Field of the Invention
The present invention is directed to the pretreatment of wastewater streams by chemical oxidation, more particularly to the pretreatment of spent caustic streams containing sulfidic, phenolic, cresylic, and/or naphthenic compounds.
2. Description of Related Art
In petroleum refining and in petrochemical processing, hydrocarbon conversion products often are scrubbed with caustic solution. In petrochemical processing, for example, such scrubbing removes hydrogen sulfide and carbon dioxide primarily as sodium sulfide, sodium carbonate and sodium bicarbonate, and also removes some of the higher molecular weight hydrocarbon constituents. Caustic solution can be used to remove naphthenic acids and other organic acids, as well as other sulfur compounds from cracked petroleum products and petroleum distillate. However, because caustic solutions are quite harmful to organic tissue, extreme care must be taken in the use and disposal of the spent caustic solutions to protect waterways, rivers, subterranean water formations, and the like. Such spent caustic solutions often are unsuitable for direct treatment in biological wastewater treatment plants because of such factors as high pH and incompatibly high levels of biochemical oxygen demand (BOD), chemical oxygen demand (COD), and total organic carbon (TOC).
Several methods have been proposed to dispose of spent caustic. Among these are wet air oxidation, chemical oxidation, and incineration. In conventional incineration processes, combustion of a fuel oil and natural gas sustains the evaporation of the aqueous parts of the waste liquor, yielding carbon dioxide and alkali metal carbonates. One major drawback with conventional incineration is that fuel oil and natural gas are relatively expensive fuel sources, especially in view of the substantial quantities of spent caustic generated during petroleum refining and petrochemical processing requiring treatment.
In Wet Air Oxidation (WAO), gaseous oxygen in the form of fine bubbles is contacted with spent caustic in contacting columns for relatively long residence times. Many devices employ steam injection as well as gas/liquid contacting devices and co-current and counter-current arrangements, with or without catalysts. One example of a WAO process is described in U.S. Pat. No. 5,891,346 to Huntley. A sulfide-containing alkaline aqueous effluent is subjected to an oxidation treatment that oxidizes sulfide ions to environmentally acceptable sulfur acid ions. The oxidation is carried out in two or more chambers connected in series. WAO processes suffer from several drawbacks, including the requirement of large capital expenditures.
Various forms of chemical oxidation also have been proposed for treating spent caustic solutions. One example is U.S. Pat. No. 5,246,597 to Jenson et al., which describes a two-step process of chemically oxidizing sulfides, such as those found in sour water from an ethylene plant, to their sulfate state. The sulfide-containing sour water is sequentially contacted with two oxidizers, hydrogen peroxide and chlorine dioxide. According to Jenson, once the sulfide is oxidized to the sulfate state, the pH of the sour water can be adjusted through addition of acids without danger of hydrogen sulfide being released into the atmosphere. The sulfates are said to remain in a water-soluble form after treatment, and optionally are subjected to further treatment. Although Jenson reports costs savings over the use of either oxidizer alone, the process is rather complex and requires close supervision.
There remains a need for a more efficient and cost effective method of pretreating spent caustic and other wastewater streams. It would be especially desirable to develop a continuous process capable of handling a wide variety of wastewater stream components as well as a wide variety of oxidizers, especially one that can be operated under a wide variety of process conditions with minimal human intervention.
According to a preferred embodiment of the present invention, a wastewater stream containing one or more compounds susceptible to treatment by chemical oxidation is pretreated by continuous chemical oxidation in a plug flow reactor. A chemical oxidizer is mixed with the wastewater stream to form a reactive mixture. The reactive mixture is flowed through a plug flow reactor suitable for reacting the compound(s) susceptible to chemical oxidation with the chemical oxidizer.
According to another preferred embodiment of the invention, a wastewater stream containing one or more compounds susceptible to treatment by chemical oxidation is continuously pretreated by chemical oxidation. A catalyst is fed into the wastewater stream at a first predetermined point of addition and mixed with the wastewater stream. A chemical oxidizer is fed into the wastewater stream at a second predetermined point of addition, which can be downstream or upstream of the first point of addition, and mixed with the wastewater stream. The compound(s) susceptible to chemical oxidation is/are reacted with the chemical oxidizer.
According to another aspect of the invention, a modular apparatus is provided for continuously pretreating a wastewater stream by chemical oxidation in a plug flow reactor. A modular unit contains means for flowing a wastewater stream and means for feeding a chemical oxidizer into the wastewater stream. The modular unit further contains a plug flow reactor suitable for reacting oxidizable compound(s) in the wastewater stream with the chemical oxidizer.
The modular apparatus optionally contains one or more modules for: (a) feeding a pH adjusting medium into the wastewater stream at one or more points of addition; (b) scrubbing vent gases present in the wastewater stream; (c) feeding a catalyst into the wastewater stream; and (d) separating acid gas and acid oil from the wastewater stream. In addition, the apparatus optionally (though preferably) includes control means for monitoring such conditions as pH, temperature, sulfide ion concentration, and/or oxidation-reduction potential of the wastewater stream at one or more locations in the apparatus. In response to the monitored conditions, parameters such as pH, temperature, pressure, chemical oxidizer flowrate, and catalyst flowrate can be automatically adjusted, as needed, to avoid or minimize the need for operation intervention.